1. Field of the Invention
The present invention relates to a planar-lightwave-circuit (PLC) technology for an optical communication component, and particularly, to a constructing method and device for an optical passive component by using an improved structure of a Mach-Zehnder interferometer.
2. Description of the Prior Art
As the information technology develops vigorously and the applications to the Internet grow rapidly, it is a must to employ the optical network having the optical fiber as the transmission medium. The specification of 10-Gbps Ethernet (10GbE) has been enacted in 2002, and the coarse wavelength division multiplexer (CWDM) is very suitable to be employed in the physical layer. Current CWDM demultiplexers (Demux) are usually made of thin film filters (TFFs) that require elaborate packaging and large module size. To reduce cost and improve reliability, an integrated PLC solution will be beneficial and can be mass-produced. Cascading directional couplers can realize the Demux, but the relatively long device limits the number of channels. To overcome this problem, the Demux using the Mach-Zehnder-interference (MZI) principle was proposed. Recently, much attention is paid to the materials of silicon-on-insulator (SOI) and silica because these materials can be low cost and exhibit <0.1 dB/cm of propagation loss in the infrared (1.3-1.55 μm) regime. However, the difference in the effective refractive index of the waveguide is relatively small, and the manufactured Mach-Zehnder interferometer will have an excessive size and bending loss problem.
In conventional design, the directional coupler is employed in the beam-splitting component for constructing the Mach-Zehnder interferometer. Although this can realize the arbitrary output of the beam-splitting ratio, the gap between the waveguides will seriously affect the output characteristics of the component. It is hard to control the gap in the manufacture process, and therefore, the yield will be greatly reduced. Because some materials for the waveguides have smaller difference of effective refractive index (Δn) so that the length difference between the two arms of the Mach-Zehnder interferometer has to be very great so as to achieve the required function of wavelength filtering. The common solution is to increase the bending radius of the curved arm. However, this will cause the bending loss, and in order to reduce the bending loss, the length of the straight arm has to be increased.